STM Fabrication and Characterization of Nanodots on Silicon
Surfaces

Using
a scanning tunneling microscope we have fabricated, characterized and modified
nanometer-sized structures on clean and oxidized silicon surfaces.Our approach involves field evaporation from
either an Al- or Au-coated tungsten STM tip.This has the advantage of allowing imaging of the structures subsequent
to fabrication, with the same tip.Application of a short voltage pulse to a tip held in close proximity to
the surface produces nanodots with a probability and dot size which depend on
the size and polarity of the pulse.

For
Au-coated tips, smaller dots are produced with negative pulses.The probability of dot formation rises
sharply above a threshold voltage, which in turn depends nearly linearly on the
oxide thickness for a constant STM set point current and voltage [1].The associated threshold field strength also
increases with oxide thickness, but shows only rough agreement with the
standard model for field evaporation.Tunneling spectra from Au nanodots show an increased probability of interface
state-associated negative differential resistivity (NDR) from those measured
above the neighboring oxidized surface, indicating that the dots act to
increase the area probed by the STM tip [2].We have also demonstrated the modification of existing nanodots, via the
application of additional, larger voltage pulses of both polarities.Negative pulses allow us to increase the
size of a dot, while positive pulses allow “erasure” of dots.